1. Field of the Invention
The present invention relates to a drive circuit for a vibration wave motor which frictionally drives a movable member by a travelling vibration wave, and more particularly to a drive circuit for causing the wave to vibrate in a stable resonance state.
2. Description of the Prior Art
FIG. 1 shows a schematic view of a vibration wave motor driven by a travelling vibration wave, which motor has recently been put into practice. Numerals 1a and 1b denote electrostrictive devices which may be made of PZT (zirconium lead titonate) and numeral 2 denotes a vibration member which is made of an elastic material on which the electrostrictive devices 1a and 1b are bonded. The vibration member 2 as well as the electrostrictive devices 1a and 1b are held on a stator (not shown). Numeral 3 denotes a movable member which is press-contacted to the vibration member 2 to form a rotor. The electrostrictive devices 1a and 1b are bonded as shown in FIG. 1B and one group of electrostrictive devices 1a are displaced from the other group of electrostrictive devices 1b by one quarter of a wavelength .lambda. of the vibration wave. The electrostrictive devices 1a in the one group are arranged at a pitch of one half of the wavelength with adjacent ones being oppositely polarized. The electrostrictive devices 1b of the other group are also arranged at the pitch of one half of the wavelength with adjacent ones being oppositely polarized. Electrode films (not shown) are formed on front and back sides of the electrostrictive devices 1a and 1b so that A.C. voltages are applied to the electrostrictive devices 1a and 1b.
In the vibration wave motor thus constructed, an A.C. voltage of Vo Sin .omega.T is applied to the one group of electrostrictive devices 1a and an A.C. voltage of Vo Cos .omega.T is applied to the other group of electrostrictive devices 1b. Accordingly, the A.C. voltages having a phase difference of 90.degree. are applied to the electrostrictive devices of the respective groups with adjacent ones being oppositely polarized. Thus, the electrostrictive element expands and shrinks. Since this vibration is propagated to the vibration member 2, it makes a bending vibration at a pitch of the arrangement of the electrostrictive devices 1a and 1b. When the vibration member 2 projects at alternate electrostrictive device positions, it recedes at other alternate electrostrictive device positions. Since the electrostrictive devices 1a are displaced from the electroctrictive devices 1b by one quarter of the wavelength, the bending vibration travels. While the A.C. voltages are applied, the vibrations are sequentially excited and propagated through the vibration member 2 as a travelling bending vibration wave.
The travel of the wave is shown in FIGS. 2A, 2B, 2C and 2D. Assuming that the travelling bending vibration wave travels in a direction of an arrow X.sub.1 and 0 denotes a center plane of the vibration member in a quiscent state, a vibration state is represented by a chain line. On a neutral plane 6, bending stresses are balanced. On a sectional plane 7.sub.1 which is normal to the neutral plane 6, no stress is applied to a crossing line 5.sub.1 of those two planes and it merely vibrates vertically. The sectional plane 7.sub.1 makes a lateral pendulum motion around the crossing line 5.sub.1. Similarly, sectional planes 7.sub.2 and 7.sub.3 make lateral pendulum motions around crossing lines 5.sub.2 and 5.sub.3, respectively.
In FIG. 2A, a point P.sub.1 on a crossing line of the sectional plane 7.sub.1 and the surface of the vibration member 2 facing the movable member 3 is a right dead center of the lateral vibration and it makes only the vertical motion. When the crossing line 5.sub.1, 5.sub.2 or 5.sub.3 is on a positive side of the wave (above the center plane 0), a leftward (opposite to the travel direction X.sub.1 of the wave) stress is applied, and when it is on a negative side of the wave (below the center plane 0), a rightward stress is applied. In FIG. 2A, the crossing line 5.sub.2 and the sectional plane 7.sub.2 indicate the former state and the stress in the direction shown by the arrow is applied to the point P.sub.2. The crossing line 5.sub.3 and the sectional plane 7.sub.3 show the latter state and the stress in the direction shown by the arrow is applied to the point P.sub.3. As the wave travels and the crossing line 5.sub.1 comes to the positive side of the wave as shown in FIG. 2B, the point P.sub.1 makes the leftward motion and the vertical motion. In FIG. 2C, the point P.sub.1 makes only the leftward motion at the top dead center of the vertical motion. In FIG. 2D, the point P.sub.1 makes the leftward motion and the rightward motion. As the wave further travels, it makes the rightward and downward motion, the rightward and upward motion and returns to the state of FIG. 2A. Through those series of motions, the point P.sub.1 makes a rotating elliptic motion. On the other hand, the movable member 3 is press-contacted to the vibration member 2, and the rotating elliptic motion of the point P.sub.1 on the vibration member 2 frictionally drives the movable member 3 in the direction X.sub.2. The points P.sub.2 and P.sub.3, and all other points on the vibration member 2 similarly frictionally drives the movable member 3.
The vibration wave motor is efficiently driven when the vibration is in a resonance state. The resonance frequency is determined by dimensions of the electrostrictive device and vibration member, the temperature and the contact pressure of the movable member.
When the electrostrictive resonator is driven, the resonance frequency of the resonator varies with a load. Accordingly, if the drive frequency is fixed, the resonator may not be vibrated in the resonance state. Therefore, in the prior art, a voltage proportional to a strength of the mechanical vibration of the resonator is fed back to an input terminal of an amplifier of the oscillator to change the oscillation frequency to follow the resonance frequency of the resonator.
However, it has not been known to adopt the above method in the vibration wave motor which drives the movable member by generating the travelling vibration wave. There are various problems in feeding the mechanical vibration of the resonator to the amplifier, such as the need for a complex circuit.